In one embodiment of an electronic lock, an electric motor rotates a threaded driver or screw to move a follower along the screw in a direction dependent upon the rotational direction of the screw. The follower is typically interconnected to a coupling member such as a clutch head. As the follower moves along the screw away from the motor, the coupling member engages a corresponding or compatibly shaped second coupling member that is interconnected to the bolt or latch mechanism of the lock. Engagement of the coupling member causes rotation of a door knob to open a door. Reversing the direction of rotation of the screw moves the follower and interconnected first coupling member away from the second coupling member, thereby disengaging the two coupling members. Once disengaged, rotation of the door knob cannot actuate the bolt or latching mechanism and open the door.
Alternatively, as is known to those skilled in the art, the door knob may be permanently interconnected to the bolt or latching mechanism, but a blocking mechanism prevents rotation of the knob and opening of the door. In this type of system, the electric motor and follower control position of the blocking mechanism. Moving the follower in one direction along the screw moves the blocking mechanism into a blocking position and moving the follower in the opposite direction removes the blocking mechanism from the blocking position which allows rotation of the door knob to open the door.
One problem with these types of mechanisms is that the follower may become stuck to an opposing surface at one or both ends of the screw as it traverses back and forth along the screw. For example, the housing of the electric motor is positioned at one end of the screw. When the follower physically moves to the end of the screw closest the motor housing, it may abut the housing or surrounding structure. Continued driving of the electric motor and rotation of the screw causes the follower to rotate and press against the surface of the housing or the surrounding structure. This creates an increasing loading force which, in turn, creates stiction between the follower and the abutting surface of the housing or surrounding structure. When called upon to reverse direction and move the follower in the opposite direction along the screw, the electric motor may be insufficiently powered or unable to provide adequate torque to free the follower from the abutting surface due to the created stiction. It should be appreciated that this problem can occur at the opposite end of the screw or anywhere along the range of travel of the follower where the follower abuts a physical restraint to its travel. For example, a surrounding structure in the form of an enclosure or housing of the lock assembly may also give rise to this problem.
One solution to this problem is to utilize a more powerful electric motor to overcome the stiction. However, this solution increases cost, may increase the physical size of the motor, and potentially does not solve the problem, as a more powerful motor may simply recreate the same issue. A further problem is that continued rotation of the electric motor once the follower has engaged a physical stop can reduce the life of the electric motor or burn out the motor. Thus, if the follower is abutting an immovable surface and the electric motor, continues to run, the motor may overheat, incur damage or burn itself out.
This latter problem has been addressed by adding sensors within the lock housing to sense the position of the follower at opposite ends of its travel along the screw. Thus, a sensor can signal the electric motor to cease operation and stop rotating the screw before or once the follower contacts an abutting surface within the lock assembly. The sensors, such as micro switches, infrared sensors or read switches, send a signal back to the motor electronics to start or stop the motor. Examples of electronic locks that utilize sensors are models WCL-7000 and WCL-7300 sold by WaferLock of Taiwan. This prevents over rotation and resulting stiction and also prevents motor burnout. However, inclusion of sensors adds to the cost, complexity and size of the system.
Sensors may also fail when exposed to humidity or bad climate conditions. These locks can be installed in a wide variety of situations with significantly varying environmental conditions from extremely cold to extremely hot, extremely dry to extremely humid, and combinations of these conditions.
There are electronic lock mechanisms that do not rely upon sensors or movement of a follower along a threaded screw. One example is a blocking mechanism model no. 4206, sold by VingCard/Elsafe of Norway. In this device, an electric motor rotates a screw to bend a “U” or “V” shaped spring-like device coupled to the screw. The legs of the spring-like device are interconnected to a movable locking plate. Depending upon the direction of rotation of the screw, bending of the spring member extends or retracts a locking plate to engage with or disengage from the locking mechanism for the door. This mechanism is dependent upon the quality of the spring member and the tolerance between the screw and the spring member.